This invention relates to the purification of 1,1-dichloro-1-fluoroethane, also designated HCFC-141b, which has been of particular interest as a replacement for chlorofluorocarbons having similar physical properties, particularly HCFC-11 and HCFC-113. HCFC-141b may be prepared by reaction of vinylidene chloride or trichloroethane with HF. Such processes are disclosed, for example, in U.S. Pat. Nos. 2,894,044 and 3,833,676.
Many by-products are formed, containing varying numbers of hydrogen, chlorine, and fluorine atoms on methane, ethane, and ethylene molecules. Some compounds are relatively harmless since their presence does not greatly alter the physical properties for which HCFC-141b is useful but others must be removed from the product. These by-products and the unreacted feed material may be separated by distillation where possible. Since vinylidene chloride is toxic, it must be removed from HCFC-141b. Vinylidene chloride has a boiling point close to that of HCFC-141b making it difficult to separate them by distillation. After distillation of the crude product, vinylidene chloride will still be present in amounts up to about 2000 ppm (wt.). If not separated, vinylidene chloride might be present in much larger amounts, say up to several percent or more of the HCFC-141b. It should be reduced to below 500 ppm according to the specifications of the Panel for Advancement of Fluorocarbon Test (PAFTII). Preferably, the vinylidene chloride should be below about 200 wt. ppm.
Dichloroacetylene is another toxic impurity. It may be present in crude HCFC-141b in amounts up to about 25 ppm (wt.) or more and should be removed to below 1 ppm to meet the specifications referred to above.
Various methods have been suggested for removing vinylidene chloride and dichloroacetylene from waste streams. For example, in U.S. Pat. No. 4,940,824 it is shown that vinylidene chloride can be removed from HCFC-141b using carbon molecular sieves and in U.S. Pat. No. 4,940,825 that dichloroacetylene also can be removed by carbon molecular sieves from HCFC-141b or from vinylidene chloride.
In U.S. Pat. No. 4,948,479 Brooks et al. disclose the use of photochlorination in the liquid phase to add chlorine to unsaturated carbon compounds, including vinylidene chloride, which can be more readily separated from HCFC-141b.
In co-pending U.S. patent application Ser. No. 07/627,070, photochlorination in the vapor phase is shown to have advantages over the liquid phase process.
Hydrogenation of chlorofluorocarbons to remove chlorine atoms has been suggested in the art. Mantell, in U.S. Pat. No. 2,697,124 disclosed the use of hydrogen and a catalyst to dehalogenate saturated fluorohalocarbons and to produce an unsaturated product. Other examples of related processes include U.S. Pat. No. 2,802,887 in which a Pd on carbon catalyst is shown to be useful for hydrogenating the olefin 1-chloro 1,2,2-trifluoroethylene to the corresponding olefin 1,2,2-trifluoroethylene. Similarly, in U.S. Pat. No. 3,564,064 Pt or Pd on carbon or alumina are shown to be useful for the same hydrogenation reaction. Such processes were not intended to remove small amounts of impurities from a halocarbon product and the reaction products are unsaturated, that is, the double bond is not hydrogenated. Thus, such processes are contrary to the objectives of the present invention as will be seen.
In WO 90/08750 saturated halocarbons are hydrogenated to remove olefin impurities. However, only fluorocarbons and fluorohydrocarbons are included as feedstocks, that is, compounds containing chlorine atoms such as the compound of interest to the present inventors, namely HCFC-141b (1-fluoro 1,1-dichloroethane) would not be included. In view of the art suggesting that chlorine atoms can be readily displaced by hydrogen it may have been believed by the inventors in the '750 application that compounds containing chlorine atoms could not be effectively purified by hydrogenation without significant loss of such compounds.
We have now found that vinylidene chloride can be removed by hydrogenation to products which can be separated easily from HCFC-141b by distillation or other means as desired.